Contribution of Brown Dwarfs and White Dwarfs to Recent Microlensing Observations and to the Halo Mass Budget

Abstract

We examine the recent results of the MACHO collaboration toward the Large Magellanic Cloud in terms of a halo brown dwarf or white dwarf population. The possibility that most of the microlensing events are due to brown dwarfs is totally excluded by large-scale kinematic properties. The white dwarf scenario is examined in detail in the context of the most recent white dwarf cooling theory which explicitly takes into account the extra energy source that derives from the carbon-oxygen differentiation at crystallization and the subsequent Debye cooling. We show that the observational constraints arising from the luminosity function of high-velocity white dwarfs in the solar neighborhood and from the recent Hubble Space Telescope deep field counts are consistent with a white dwarf contribution to the halo missing mass as large as 50%, provided there is (1) an initial mass function strongly peaked around ~1.7 M_☉, and (2) a halo age older than ~18 Gyr.